Vehicle damage from low speed collisions during parking and crowded traffic conditions has a major effect on the costs of insurance premiums and vehicle ownership. Federally mandated regulations limiting vehicle damage during low speed collisions have required manufacturers to provide energy absorbing systems as standard equipment in new passenger vehicles. These systems have affected vehicle appearance and have resulted in significant increases in vehicle weight and cost. In addition, vehicle complexity has been increased with current systems which require unique energy absorbers to accommodate differences in vehicle weight and strength of vehicle structures.
During a collision, the kinetic energy of a vehicle is dissipated with an energy absorption system by the work of displacing the vehicle's bumper inwardly with respect to the vehicle structure against the resistance force of an energy absorber. An optimum energy absorption system would provide a constant resistance force to the inward displacement of the bumper. The constant resistance force would minimize the required inward displacement of the bumper and the forces acting on the vehicle structure. By necessity, the maximum force developed by an energy absorption system must be held below the failure point of the vehicle structure reacting the absorber force.
Energy absorption systems in the prior art consist primarily of two types, namely, elastic and hydraulic systems. The elastic system is a linear system in which the resistance force to bumper inward displacement is produced by deflecting an elastic device and varies directly with bumper displacement.
Elastic systems include bumpers having rubber and other elastic materials and bumper mountings which rely on rubber elements which deflect during vehicle collisions. The major disadvantage of elastic systems is that they absorb only one-half the energy of the ideal constant force system. As a result, large bumper displacements or high absorber forces are required to dissipate the kinetic energy of a low speed vehicle collision. Another disadvantage is they are heavy in weight. Another disadvantage is that they tend to be relatively large and difficult to adapt to vehicle designs.
Hydraulic systems utilize hydraulic absorbers which are similar in principle to vehicle suspension shock absorbers. Energy is absorbed by forcing a hydraulic fluid through an orifice. The hydraulic absorber is a more efficient energy absorber than the elastic absorber but more complex and higher in cost. A further disadvantage of the hydraulic absorber, as compared to the ideal constant force absorber, is that the force which the absorber produces is a function of impact velocity. This means that larger bumper displacements occur at impact speeds less than the speed, corresponding to the maximum absorber force, as determined by the limiting strength of the vehicle structure than with the ideal constant force absorber. Large bumper displacements are undesireable because of the increased possibility of vehicle damage.
With the foregoing in view, an effective, compact, energy absorber, lower in cost than the hydraulic absorber, capable of developing a constant force over the range of vehicle impact speeds for which damage protection is sought would provide improvements over the prior art. Moreover, further benefit would be provided if the constant force of the absorber could be adjusted by the manufacturer to accommodate a range of vehicle weights and limits of structural strength.